1. Field of the Invention
The present invention relates to organic thin film solar cells having a thin film in which a light-receiving layer is formed at a junction between two dissimilar organic semiconductors.
2. Description of Related Art
In order to use natural energy resources more efficiently, solar photovoltaic systems are being actively developed. Most of today's solar cells for photovoltaic generation use single- and poly-crystalline silicon as a starting material. However, they have not yet been put into widespread commercial use because of such problems as unstable supply of crystalline silicon and difficult-to-reduce relatively high manufacturing costs.
In this situation, solar cells which do not use crystalline silicon are being developed. Such known solar cells include: CIS solar cells which use a thin film of a compound semiconductor containing copper (Cu), indium (In), selenium (Se), etc.; dye sensitized solar cells that generate electricity by an electrochemical mechanism provided by a combination of a metal oxide such as a titanium (Ti) oxide and an organic dye; and organic film solar cells that form a junction between a conjugated polymer and an electron-accepting molecule by an appropriate method. These solar cells have the advantages of stable material supply and a relatively simple manufacturing process. In particular, organic thin film solar cells, having an overall thickness of 1 μm or less, offer advantages in applicability to flexible solar cells (such as lightweight and flexibility), and also have an advantage in simple manufacturing.
A basic structure of an organic thin film solar cell will now be described. FIG. 9 is a schematic cross-sectional view illustrating a principal structure of a planar heterojunction organic thin film solar cell. The organic thin film solar cell of FIG. 9 includes: a transparent electrode 111 which is formed by stacking a transparent conductive film 102 (e.g., tin-doped indium oxide [ITO]) on a transparent substrate 101 (e.g., a glass substrate); a light-absorbing dye 501 (e.g., a copper phthalocyanine and a conjugated polymer) stacked on the transparent conductive film 102; an electron-accepting molecule 1002 (e.g., a fullerene derivative) stacked on the light-absorbing dye 501; and a rear electrode 106 (e.g., aluminum [Al]) stacked on the electron-accepting molecule 1002. In such organic thin film solar cells, the photoelectric conversion reaction is confined to the junction plane between the light-absorbing dye 501 and the electron-accepting molecule 1002. So, it is generally accepted that such a two-dimensional planar junction as shown in FIG. 9 can produce only a relatively small amount of photogenerated charge because of the above-mentioned limited junction (reaction) area, thus resulting in poor photoelectric conversion efficiency.
Under this circumstance, in order to increase the photogenerated charge of an organic thin film solar cell, attempts are being made to increase the junction area between the light-absorbing dye and the electron-accepting molecule. FIG. 10 is a schematic cross-sectional view illustrating a principal structure of a bulk heterojunction organic thin film solar cell. In the bulk heterojunction organic thin film solar cell of FIG. 10, a solution of a mixture of a light-absorbing conjugated polymer 1001 (e.g., 2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylen) and a fullerene derivative 1002 is applied on a hole transport layer 103 by an appropriated method to form a thin film of the mixture. In this case, the conjugated polymer 1001 and the fullerene derivative 1002 are phase separated in the mixture film, thus creating multiple junctions between the thus separated two phases. Such a structure is considered to be able to provide a larger junction area than structures in which the junction is provided by stacking planar films (e.g., see Nonpatent Document 1).    Nonpatent Document 1: Christoph J. Brabec, N. Serdar Sariciftci, and Jan C. Hummelen: Adv. Funct. Mater. 2001, 11, p. 15.
As described above, in bulk heterojunction organic thin film solar cells, a mixture of a light-absorbing conjugated polymer and a low-molecular-weight, electron-accepting organic compound molecule is phase separated to form multiple junctions between the two materials. Such a structure is considered to increase the junction area and therefore increase the photogenerated charge, thus leading to an increase in photoelectric conversion efficiency.
As for the charge generation mechanism of an organic thin film solar cell, it is usually explained by extraction of electrons by the electron-accepting molecule from the light-absorbing dye which has absorbed light. In order to achieve a high efficiency organic thin film solar cell, design and application of a molecule with strong electron-accepting properties is necessary. At present, molecules satisfying such excellent electron-accepting properties are limited to only a few materials such as fullerene derivatives and perylene derivatives. Unfortunately, these electron-accepting molecules exhibit only relatively weak electron-accepting properties and can therefore produce only a limited amount of photogenerated charge.
In this situation, in order to obtain materials having improved electron-accepting properties, efforts are being made to use, as an electron acceptor, inorganic nanoparticles which have more excellent electron-accepting properties than the aforementioned fullerene derivatives and perylene derivatives. Inorganic nanoparticles typified by CuIn nanoparticles have more excellent electron-accepting properties than organic nanoparticles. However, any attempt to use such inorganic nanoparticles in a bulk heterojunction organic thin film solar cell has so far failed to improve the photoelectric conversion efficiency. In order to make effective use of inorganic nanoparticles as the electron acceptor of a bulk heterojunction organic thin film solar cell, the inorganic nanoparticles need to assume a percolation structure in which they are beaded. To achieve such a percolation structure, inorganic nanoparticles need to be contained in a high concentration, typically more than 40 wt %. However, organic thin film solar cells are thin, so excessive incorporation of such inorganic nanoparticles can cause such problems as degradation in the light absorbing properties and short-circuiting of the thin films of the cell, thus hampering improvement in the photoelectric conversion efficiency. As described above, it is generally difficult to achieve the percolation structure of inorganic nanoparticles in an organic thin film solar cell without sacrificing other important properties.